There is known a ball screw having balls capable of rolling between a screw shaft and a nut. Such a ball screw has been in the actual use in a positioning mechanism of a machine tool/robot, a feeding mechanism, a steering gear of a vehicle and the like as it allows reduction of the friction factor in rolling the screw shaft relative to the nut, as compared with a sliding-contact type screw.
In these years, in order to increase an allowable load, there has been devised a roller screw using rollers as rolling elements instead of balls, which is disclosed in the patent document 1, for example. In such a roller screw, a screw shaft has a roller rolling surface spirally formed on the outer surface thereof and a nut also has a roller rolling surface spirally formed on the inner surface thereof to face the roller rolling surface of the screw shaft. In a rolling path formed between the roller rolling surface of the screw shaft and the roller rolling surface of the nut, there are arranged plural rollers as rolling elements. When either the screw shaft or the nut is rotated, the rollers move (revolve) around the screw shaft while rotating on their axes.    [Patent Document 1] Japanese Patent Laid-open Publication No. 11-210858